Wire cut electrical-discharge machining apparatus

ABSTRACT

An apparatus for preventing damage and malfunction to workpiece and electrode travelling system by quickly and reliably detecting wire electrode breakage during the wire cut discharge machining and, in the event of breakage, stopping machining or the like. Unlike conventional mechanical detecting means and devices for detecting the severing of a wire electrode from its supply reel, the present apparatus detects the position of the spark discharge pulse along the axial direction of the wire electrode to determine whether the location is inside or outside the thickness of the workpiece. If the spark position is outside the workpiece thickness and continues for a predetermined time period, the apparatus determines that a wire breakage exists. In addition, the apparatus attempts to discriminate a wire electrode breakage from other malfunctions, especially when the spark position is detected as occurring outside the workpiece thickness, and if a feed member wear situation is detected, the position of the sliding contact of the feed member can be updated.

TECHNICAL FIELD

The present invention relates to a technique for detecting the positionalong the axial direction of a wire electrode of an electrical-dischargewire cutting machine ("WC-EDM") where spark discharges are generated.The invention further relates to a wire breakage detection apparatus andapparatus for detection of the wear of upper and lower electrical feedmembers.

BACKGROUND ART

A typical wire cut electrical-discharge machining apparatus uses a thinwire of about 0.05 to 0.35 mm in diameter as an electrode tool. The wireis disposed under a predetermined tension between a pair of guidemembers in such a manner that the portion of the wire in the workingzone can be renewed by axial displacement while maintaining the wireunder tension.

A contour may be cut in the workpiece to be machined by moving theworkpiece with respect to the wire electrode and in a direction which isgenerally orthogonal to the axis of the wire electrode, whilemaintaining a minute gap between the wire and the workpiece and underconditions wherein a working fluid is introduced into the gap betweenthe wire and work piece. Contour machining is carried out by impressingvoltage pulses across the gap between the wire and the workpiece andmoving the workpiece with respect to the wire electrode in order toestablish a work feed in the desired machining direction.

However, unless suitable operating conditions are maintained duringmachining, wire breakage is apt to interrupt production.

Recently, WC-EDM's have been available which have included aself-recovery means for dealing with wire electrode breakage. Morespecifically, WC-EDM's have been provided with functions forautomatically inserting and joining the wire electrode ends. However,these arrangements do not necessarily assure 100% effective or reliableautomatic wire electrode insertion and joining. Further, even if arepair is appropriately achieved, the rethreading function not onlywastes time but also generally impairs machining accuracy, finish, andthe like. Hence it is preferable in the first instance to avoid thecauses of wire breakage.

When wire cut electrical-discharge machining, a number of problems whichtend to cause wire breakage may be encountered. It is usually impossibleto carry out high speed machining with accuracy and high efficiencyunless high-load working conditions, and in particular electricalconditions such as voltage pulse width, off time duration and amplitudeof the discharge current, and machining feed control, are establishedand controlled. However, when machining under high load conditions,there is a higher risk of wire breakage. In addition, for precisionmachining, other conditions, such as high wire tension and the like, arerequired which exacerbate the danger of wire breakage.

Therefore, some recent WC-EDM's have included devices to carry outvarious detection and control strategies in order to prevent wireelectrode breakage. Further in the event of wire electrode breakage,some recent WC-EDM's have included devices to quickly detect the breakand, thereafter, execute a control function which at least temporaryarrests the supply of machining voltage pulses or power, or arrests thetravelling (renewal) of the wire electrode, or temporally arrests themachining action including cutting off the work fluid supply, machiningfeed and the like. This is done to prevent malfunctions and theconsequences of such abnormal failures on the workpiece, the workingfluid nozzle, the supply and travel systems for the wire electrode andthe like in the event of wire electrode breakage.

However, the foregoing broken-wire detection systems are not necessarilyas quick as desired. Accordingly, it has been difficult to avoidobstacles to machining progress and accuracy due to excessive detectionand control, and problems involving different failures which result fromdelays in adaptive control.

There are various type of wire electrode broken-wire detectors orbroken-wire detecting control devices already available for conventionalwire cut electrical-discharge machining apparatus. However, most ofthese devices are of the type wherein discrimination is based ondetection on the outboard side of the supply side guide member, i.e.,between the supply side guide member and the supply reel and at alocation such as along the wire feed path along or at structurespositioned between the supply reel and the supply side guide member overwhich the wire is strung, and/or on the outboard side of the take upside guide member and used wire recovery member. Typical devices usesensors with contact(s) such as limit switches and photodiode sensors,or operate to detect the tension and/or change in travelling speed ofthe wire electrode.

However, as most wire breakage occurs proximate the workpiece andbetween the upper and lower guide members, the indirect detection at theoutboard sides of the interspace between the guide members has resultedin a problem in that accurate detection by the various sensors isdelayed.

As disclosed in JP-A-53-68496 for example, it is known to arrange acurrent source to supply a minute current, which is not directly relatedto the working current, to the wire electrode from feed members disposedabove and below the work table on which the workpiece is fixed, and touse a current detector to detect any fluctuation of the minute current.This permits detection of a break in the electrode wire by detectingwhen the minute current drops to zero.

However, the above broken-wire detector is so arranged that minutecurrents are by necessity supplied from the current source to the feedmembers above and below the workpiece. Therefore, it exhibits thedisadvantage of being in contact with the machining power supply by wayof the upper and lower feed members and the workpiece. The minutecurrent flows from the current source through the incoming line formachining power at the time of breaking of the wire electrode. Thisinhibits positive detection of a wire break condition.

In addition, with this type of broken-wire detector, in some cases it ispossible that for a short time after a wire break occurs, both free endpieces of the wire in the vicinity of the break will make repeatedcontact with locations on the workpiece and with other electricallyconductive material adjacent to the wire electrode guide path, thuspreventing the minute currents from being immediately and completelycut-off. This often results in delayed detection of a wire break.

In addition, as above described, the present invention relates to animprovement in detection of wear of one or both of the feed members.This is because, in the case where wire breakage detection is carriedout using a method where detection is accomplished through theelectrical power supply elements using the electrical power supplycircuit, it can by-pass the above mentioned wear detection means.

By way of example, JP-A-60-108226 discloses a wire cut electricaldischarge machine comprising a plurality of feed members which areprovided with diodes which causes electrical current to flow in the samedirection. These feed members are connected to a power source, of eitherdirect or alternating current, of a predetermined magnitude. Theelectricity from the power source is supplied by way of an impedanceelement and wire breakage is detected by comparing the voltage acrossthe ends thereof with a reference value.

In JP-A-63-109915, an electrical-discharge machine is disclosed whichincludes a circuit which supplies electric energy to the workpiece fromthe feed members, a wire electrode break detecting circuit to detect abreak of the wire electrode, means to detect the current flowing fromthe electrical-discharge machining power circuit into the wire electrodebreak detecting circuit when wear of the feed members occurs, and meansto detect the machining speed. With this arrangement, the valuesdetected by the current detecting means and machining speed detectingmeans are compared with respective standard values and the time forreplacement appropriately indicated.

However, both of these wire electrode break detecting means suffer fromdelayed detection in that both working current and any separate current,both flow through the wire electrode. Further, as the feed members wear,the electrode break detecting means, and similar types of detectionmeans and arrangements which are associated with the establishment ofelectrical contact by the electrical feed members, suffer impaireddetection accuracy.

In addition, in JP-A-4-129617, an electrical-discharge machine isdisclosed wherein the existence of an abnormality is determined on thebasis of the detected voltage, and includes means for detecting thevoltage between the wire electrode and the feed members. However, inorder for the voltage at a location immediately adjacent the feedmembers to be detected, it is necessary to provide a contact which isseparate from the feed members and used only for the detection of thewire electrode voltage. This not only leads to a complicated and costlyarrangement, it also leads to a situation wherein an abnormality due todetecting conditions at the contacts may occur. This of course rendersaccurate detection unlikely.

OBJECTS AND SUMMARY OF THE INVENTION

In view of the above drawbacks, it is a general object of this inventionto provide an arrangement which, in order to detect the wire electrodebreak at the location where the wire is actually effecting machining andin portions proximate thereto, every time a spark discharge pulseoccurs, the position of spark discharge in the axial direction of thewire electrode is detected (preferably with a high degree of accuracy),and which is capable of distinguishing between situations where theabove-mentioned spark-discharge position is located within a workpiecefrom situations where the spark-discharge position is outside theworkpiece. Precise and reliable wire electrode break detection isquickly achieved based on the frequency with which spark-dischargepositions occur outside the workpiece.

It is a further object of this invention to provide an arrangementwhich, when the spark-discharge positions outside the workpiecethickness are detected, and no signal indicating wire break is generatedwithin a predetermined time, issues a signal indicative of electricalsupply contact wear. In response to this wear indication, adetermination is made as to which of the upper or lower feed members isworn based on the location (i.e., outside the workpiece) where the sparkdischarge pulse occurred.

The present invention also provides a technique for detecting wireelectrode break and wear of feed members, as noted above. This isachieved by detecting the spark-discharge positions in the axialdirection of the wire electrode in between the upper and lower feedmembers, each electrical feed member being positioned on one side of theworkpiece. In other words, on the basis of machining voltage pulsessupplied to the wire electrode through the feed members from oneterminal of the machining power supply, it may be determined whether theelectric energy has travelled to some position outside the work piecethickness, or on the other hand, back to the other terminal of themachining power supply after flowing through intervening materialadjacent to the material to be machined. By processing the signals fromthe above detected spark-discharge positions, it is possible todetermine both wire breakage situations and feed members wear.

The state of the technology to detect a spark-discharge position in wirecut electrical-discharge machining can be summarized as follows:

JP-A-53-64899 discloses a voltage measuring circuit which detectsfluctuations of electrical resistance between sparking points, feedmembers and the workpiece, with respect to the wire electrode bymeasuring the voltage across the same. This enables the spark-dischargepositions to be measured or detected.

JP-A-59-30621 discloses that by measuring the current value flowing intoeither one or both of the upper and lower feed members, aspark-discharge position can be detected through a current waveformwhich is a function of inductance at the sparking point. Further, fromJP-A-62-15017, it is known that currents flowing to the wire electrodethrough the upper and lower electrical feed members may be detected byindividual current comparators, and that a signal representative of thedifference between the currents flowing into both current comparatorsmay be derived via differential amplification, to thereby indicate aspark-discharge position.

Even though the invention which is disclosed hereinafter can make use ofsuch prior art if desired, it is preferable for the spark-dischargeposition to be detected using detection signals, as describedhereinafter, which provide even higher accuracy.

In order to achieve the above as well as other objects and advantages, awire cut electrical-discharge machining apparatus according to a firstinventive aspect of the present invention may comprise a wire electrodewhich is supplied on a renewing feed basis in the axial direction underconditions wherein the wire electrode extends between upper and lowerguide members and upper and lower feed members which are respectivelydisposed on either sides of the workpiece to be machined, and whereinmachining is carried out by generating spark discharge pulses byimpressing voltage pulses on a periodic basis across the machining gapformed between the workpiece and the wire electrode while a workingfluid fills the machining gap. The position where spark discharges occurmay be determined using a sampling of the electrical discharge machiningpulses. The arrangement preferably includes a current comparator fordetecting the current flowing into one or both of the electrical feedmembers due to spark discharges generated from the application of themachining voltage pulses, a spark-position detecting means foramplifying output signals from said current comparator(s) and forissuing a signal indicating the position along the electrode wire,between the pair of feed members, at which the discharges occurred, adigital data arithmetic unit for converting the signal from said sparkposition detecting means into digital data indicative of the sparkdischarge position, and a setting device which, when a discharge isgenerated at or about the center position along the axial direction ofthe wire electrode, uses the digital values generated by the digitaldata arithmetic unit as standard upper and lower values of platethickness suitable as settings for the upper and lower limits for thespark discharge position.

Further, there may be included a workpiece thickness inside/outsidedetector for spark discharge positions which outputs a signal indicativethat a spark is outside workpiece thickness when the digital datacorresponding to the spark generation location, which is output from theabove-noted arithmetic unit upon a generation of spark discharge pulsein the machining gap, is outside the digital values e.g. above the lowerlimit and below the upper limit, set by the upper limit and lower limitadjusters. A broken-wire-electrode discriminator, which outputs a wireelectrode break signal when it is determined that the spark dischargeposition is outside the workpiece thickness, is also preferablyincluded.

In order to increase the accuracy of the spark discharge positiondetection, it is preferable to provide, in the broken-wire-electrodediscriminator, an arrangement comprising: upper and lower peak holdingcircuits for holding the peak currents detected by the upper and lowercurrent comparators each time a spark discharge is generated, aspark-position detecting means in the form of a difference amplifier,for determining the difference between the peak currents latched by thepeak current holding circuits to thereby produce a signal relating tothe spark position, and in order to convert the output values from thedifference amplifier into digital data representing information relatingto the spark discharge position within an extremely short period of timeafter the termination of the gate signal, a time delay circuit for thegate termination signal may be advantageously provided in the digitaldata arithmetic unit for determining spark discharge positions.

In addition, it is preferable to arrange the broken-wire-electrodedetection device so that in use, the accuracy of detecting the sparkdischarge positions does not decrease or vary as the predeterminedelectrical machining conditions are changed. When the spark positionsignal obtained through the difference amplifier is supplied to thearithmetic unit, those signals are converted into digital dataindicative of the spark discharge position. The degree of amplificationof the circuit for amplifying the spark position signal may varyaccording to the predetermined machining conditions such as thoserelating to the width or duration of a machining pulse, and it ispreferable to allow for alteration of that amplification, such as by asetting.

In addition, as the preferred embodiment of the device used as the wireelectrode breakage discriminator, it is preferable to include a sparkposition inside/outside the workpiece thickness detector for outputtinga signal indicative that the spark position is within the workpiecethickness in the event digital data indicative of discharge positionfrom the arithmetic unit is within the above-noted digital data values,i.e., less than the upper limit and more than the lower limitestablished by the upper and lower limit adjusters. The wire electrodebreakage discriminator may also preferably include a counter forcounting the position signal indicative of whether the spark isinside/outside the workpiece thickness, the counter being arranged sothat the spark position signals indicative of a spark position outsidethe workpiece thickness are integrally counted. The integrated count ofthe outside-the-workpiece thickness spark position signals is cleared bythe input of an inside-the-workpiece thickness spark position signal,and a broken-wire-electrode signal is output to the wire cutelectrical-discharge machining control device when the integrated countof the outside-the-workpiece thickness spark position signals reaches apredetermined value.

Moreover, in accordance with another aspect of the present invention, awire cut electrical-discharge machining apparatus is provided whichcomprises, in the forgoing first device, a further device for generatingan electrical feed member wear detecting signal when the number ofoutside-the-workpiece thickness spark position signals reaches a presetvalue which is significantly smaller than the value set for the wireelectrode breakage discriminator. Such detection is done in accordancewith the output of the detection signal relating to theoutside-the-workpiece thickness spark position signal, which is outputby the inside/outside workpiece thickness position spark detection. Thedistinct feature of this aspect of the invention is a device whichoutputs an electrical feed member wear discrimination signal, if, afterthe input of said wear detection signal, no wire electrode breakagesignal is output by the wire electrode break discrimination devicewithin a preset time period.

Further, with respect to the second aspect of the invention, it ispreferable to provide an arrangement comprising: a logic circuit whichgenerates an upper or lower wear detecting signal using the outsideworkpiece thickness spark position signals and the feed member weardetecting signal output by said inside/outside workpiece thicknessdetector, and a device for outputting an upper or lower weardiscrimination signal to a display associated with the wire cutelectrical-discharge machining control device and to provide a warningby means of the wear discriminating signal and said logic circuit signalfor the above-stated electrical feed member wear discriminator.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the presently preferred embodiments of thepresent invention and, together with the description, serve to explainthe principles of the invention. In the drawings:

FIG. 1 is a block diagram representing the general arrangement of afirst embodiment of the present invention;

FIG. 2 is an illustration of a connecting circuit for the parts of apower supply 7A and a control device 7 for the WC-EDM of FIG. 1;

FIG. 3 is an illustration of a peak holding circuit as an example of theholding circuit of FIG. 1;

FIG. 4 is an illustration of the circuit connections for thedifferential amplifier of the spark-position detecting means 11 and thedigital data arithmetic unit 12 for detecting spark discharge positionsin FIG. 1;

FIG. 5 is an input/output voltage and current waveform and timing chartfor individual signals from the circuits of FIGS. 1 through 4; andcolumns A, B, C and D represent cases where a gate signal G2 differs inwidth as well as other differing discharge conditions;

FIG. 6 is a waveform and timing chart similar to that shown in FIG. 5,illustrating the case wherein the peak holding circuit is used;

FIG. 7 is a time chart plotting digital data values PD indicative ofspark discharge position distribution under stable machining operationsover time;

FIG. 8 is a time chart plotting digital data values indicative of aspark discharge position distribution under conditions whereinshort-circuiting or a spark discharge position concentration isoccurring over time;

FIG. 9 is a block diagram representing the general arrangement andinterconnective relationship of the inside/outside workpiece thicknessspark position detection device wherein digital data relating to thedetected spark position is used to ascertain wire electrode breakage inaccordance with the frequency of detection of the outside spark positionsignal detection;

FIG. 10 is a longitudinal sectional view of one embodiment of an upper,and lower water supply and guide block incorporating electrical feedmembers;

FIG. 11 is a cross sectional view of an arrangement for moving the upperand lower feed members illustrating a specific example of the associateddriving mechanism;

FIG. 12 is a block diagram illustrating the general arrangement of oneembodiment for implementing electrical feed member wear detectionaccording to a second embodiment of the present invention; and

FIG. 13 is a block diagram illustrating an example of a circuit for weardetection and wear discrimination.

DETAILED DESCRIPTION OF THE INVENTION

A presently preferred of the present invention will now be described.

FIG. 1 is a block diagram illustrating first and second embodiments ofthe present invention. In this figure a wire electrode 1 is renewablyfed in its axial direction under conditions wherein a predeterminedtension is applied to the portion of the wire which extends between apair of spaced positioning guides which are separated by a predetermineddistance. Machining is carried out by moving a workpiece 2 in adirection essentially normal to the axial direction of the electrodewire 1 until it is spaced from the electrode wire by a very small gap g.Voltage pulses are supplied to and impressed across the gap between theworkpiece 2 and the wire electrode 1 from a wire cutelectrical-discharge machining power supply 7A (FIG. 2) by way of upperand lower interpole lines 3A and 4A and upper and lower electrical feedmembers 3 and 4 under the conditions wherein a working fluid is ejectedfrom a nozzle 5 and induced to flow in the gap g. This generatesperiodic sparking pulses. Relative machining feed is provided in thehorizontal plane at right-angles to the axial direction of the electrodewire through a feeding mechanism 6 having X and Y axis feed motors Mxand My which produce movement in the X-Y plane. Reference numeral 7Brepresents a machining feed driving circuit, reference numeral 7Crepresents a working fluid supply circuit, and reference numeral 7represents a wire cut electrical discharge machining control deviceincorporating a NC unit which carries out settings, changes and controlsfor the power supply 7A and circuits 7B and 7C based on establishedworking conditions, programs and various control signals.

Reference numerals 8 and 9 respectively denote current detectors whichmonitor the electrical discharge current flows which pass through theupper and lower feed members 3, 4 to the wire electrode as theabove-mentioned machining discharge pulses are generated.

Reference numeral 10 denotes a holding circuit which is usually providedwith a sampling circuit, which is preferably provided to receive theinputs from the current sensors 8, 9, and which, by way of example, maybe used to latch the current signal which is produced in synchronismwith the termination of the machining pulse gate signal.

In accordance with the present invention, the presently preferred modeof practicing the above-mentioned hold circuit 10 is to use a peak holdcircuit (FIG. 3) which latches to the maximum value of the detectedcurrent signal.

Reference numeral 11 denotes a spark position detection means whichamplifies the output of the machining current detectors 8, 9 or theoutput of the peak held values for detecting the position along the wireelectrode between the feed members 3, 4 at which a spark occurred.

In the case where a pair of upper and lower current comparators 8 and 9are used, which is the more desirable mode of carrying out the presentinvention, an arrangement including a difference amplifier (FIG. 4) isprovided for differentially amplifying both detected signals or heldsignals.

Reference numeral 12 denotes a digital data arithmetic unit (FIG. 4) forconverting the spark position signal, which is indicative of the sparkdischarge positions and which has been amplified by said spark-positiondetecting means 11, into digital data, e.g., eight-bit digital data.

Reference numeral 14 denotes a memory for storing the digital data PDwhich is indicative of the spark discharge positions output by saidarithmetic unit 12

Reference numeral 13 denotes a PC with printer for carrying out therequired operation and processing of the digital data stored in saidmemory 14 in response to the operator control and for generating hardcopies if desired.

Reference numeral 15 denotes inside/outside workpiece thickness detector(FIG.9) for determining, in accordance with the digital data PDdelivered from said arithmetic unit, whether the spark dischargepositions are within a thickness of the workpiece 2 during machining; inother words, whether discharge machining is occurring to material otherthan the intended workpiece.

Reference numeral 16 denotes a broken-wire discriminator (FIG. 9) whichissues a broken-wire-electrode signal (DANSEN) whereby said wire cutelectrical-discharge machine at least stops its machining motion in theevent that a count in excess of predetermined level occurs, e.g., thesignals produced by the inside/outside workpiece thickness detectorindicative of a spark occurring outside the workpiece thickness exceedsa predetermined number.

Reference numeral 7 denotes the control device which receives abroken-wire-electrode signal(DANSEN) from the discriminator 16, and atleast stops supplying the working voltage pulses generated by themachining power supply 7A to thereby stop machining. Alternatively, as afurther measure, the wire electrode feed can be temporarily stoppedand/or the operation of the control device 7 stopped.

In FIG. 2, reference numeral 7A denotes a wire discharge machining powersupply pulse control for a two-power source supply, one for producing ahigh voltage, at a low amperage current, and another for producing a lowvoltage at high amperage current. Source V1 is an auxiliary power sourcefor supplying the high voltage, small amperage current; e.g., a voltageof about 300 volts. Source V2 is the main machining power source which,as compared with the auxiliary source V1, produces about 100 volts. Itis the source of the low voltage, high amperage current. Diodes D1 andD2 are protective diodes which prevent reverse current flow. Resistor Ris a current flow control resistor. Transistors TR1 and TR2 are ON/OFFswitching elements for said auxiliary supply V1 and main supply formachining V2, respectively. Signals G1 and G2 are gate signals which aresupplied to the switching elements TR1 and TR2, respectively.

The gate signals G1 and G2 are input to the gate electrodes of therespective switching elements TR1 and TR2 which preferably consists ofMOS.FET type transistors. The gate signals are generated by the pulsecondition setting and control section 7a of the machining power sourcecontrol 7A which in turn is set and controlled by the wire sparkmachining control device 7 which also encompasses the above noted NCcontrol device.

Accordingly, voltage or spark pulses are supplied between the wireelectrode 1 and workpiece 2 with desired polarity, voltage or sparkingduration, quiescent time, and current value.

FIG. 3 illustrates an example of a preferred embodiment of a peakholding circuit for the sample and hold circuit 10 for the detectedsignals from the current detectors 8 and 9. The peak holding circuit 10outputs a spark position signal to the spark position detecting means 11after amplifying the detected current signals.

The illustrated embodiment comprises upper and lower peak holdingcircuits 17 and 18 for the current detectors 8 and 9, respectively,which contemplates detection at both the upper and lower electrical feedelements as a preferred embodiment for carrying out the presentinvention. The above noted peak holding circuits 17 and 18 each compriseholding capacitors 21, 25, two operational amplifiers 19/20 and 23/24,and reset transistors 22 and 26, wherein reset signals are entered fordischarging held charges on the capacitors 21 and 25, respectively,thereby resetting the circuits.

FIG. 4 illustrates a circuit diagram of an embodiment of the sparkposition detecting means 11 in which the holding circuits amplify thecurrent signals detected by the current detectors 8 and 9, or latch thedetected current signals and then amplify the held signals to deliver aspark position signal according to the detected spark position. FIG. 4further includes, in the form of a block diagram, the digital dataarithmetic unit 12 for generating a spark discharge position signal. Asan embodiment of the detecting means 11, FIG. 4 illustrates, as anexample, the use of a differential amplifier 27 as the amplifierpreferred in the case wherein current detection is done at both theupper and lower electrical feed members, this being the preferred modeof carrying out the invention. In the differential amplifier 27,operation amplifiers 28 and 29 receive upper and lower detected currentsignals I_(UP) and I_(DW), respectively, and the outputs of theamplifiers 28 and 29 are input to a differential amplifier 30 andamplified after regulating the difference (I_(UP) -I_(DW)) between thedetected current signals [see FIG. 5(f)]. The difference signal isvirtually zero when a spark discharge takes place at the midway positionof the wire electrode 1, i.e., midway between the upper and lower feedmembers 3 and 4 and against the workpiece 2. In this case, a sparkposition signal VC is delivered. The arithmetic unit 12 receives thespark position signal VC and converts the signal to digital format byA/D conversion. Thus, the analog spark position signal corresponding tothe difference between the detected current signals is output in digitalform by converting the analog signal into digital form, e.g. into aneight-bit digital data PD. In this embodiment, the spark position signalVC is arranged so that the signal VD, regulated to the workingconditions, is input to the arithmetic unit 12 through an amplifiercircuit having an amplification factor which varies inversely withrespect to certain machining condition parameters such as machiningvoltages or spark pulse width (time).

The amplifier circuit may comprise an operation amplifier 31 forinverted amplification of the signal VC, a transistor TR3 inserted in afeedback circuit of the amplifier 31 to convert and regulate theamplification factor, and an A/D converter 32 for supplying a signal tothe transistor TR3 so that the amplification factor of the amplifiercircuit varies inversely with respect to predetermined machiningconditions such as the machining voltage or spark width. The gatesignals supplied to the transistor TR3 are counted and an analog signalis output. The reason the amplifier circuit is so arranged is that withwire cut electrical discharge machines and the many electrical machiningparameters involved, variation in the possible current amplitude, whichis set or is a consequence of the parameter setting, depends to a largeextent on the set or desired voltage, spark pulse width and/or time. Theresult is that the waveform of the spark current will be generallytriangular and as a rule the spark current will not rise to or exceedthe predetermined value under the voltage and spark width settingsusually set as machining conditions. Therefore, as described inconnection with FIG. 5, by differentially amplifying the differencebetween the above detected current signals (I_(UP) -I_(DW)) with a sparkposition detecting means 11, without more, it may not be possible todistinguish between detected spark positions for different voltage orspark width settings.

FIGS. 5 and 6 are charts illustrating signals such as input/outputvoltages, current waveforms and timing signals for individual blocks ofthe block diagrams and circuits of FIGS. 1-4. FIG. 5 illustrateswaveforms present in the embodiment where the holding circuit 10 is notused, while FIG. 6 shows waveforms present in the embodiment where peakholding circuits 17 and 18 in FIG. 3 are used as the holding circuit 10.

FIG. 5 columns A, B, C and D illustrate different cases of the voltageor sparking pulse gate signal. In columns A, C and D the gate signals,G2, row (b), have the same pulse width; i.e. for columns A, C and D(S1=S3=S4). Column A represents the case where the current differencesignal, row (f) (I_(UP) -I_(DW))=0.

Column C represents the case where the current difference signal resultsin a positive value. Column D represents the case wherein the currentdifference signal results in a negative value. Column B represents thecase where the gate signal, G2, row (b), is less than S1 (=S3=S4). Inother words, the width of the machining pulse is narrow, such as may bethe case where conditions are set for higher precision machining.

In FIG. 5 the signal G1 of row (a) illustrates the gate signal forswitching element TR1; signal G2 of row (b) illustrates the gate signalfor the switching element TR2; the signal IG of row (c) illustrates thewaveform of the spark current (I_(UP) +I_(DW)); signal I_(UP) of row (d)illustrates the upper detected current signal flowing from the sparkpoint to the workpiece 2 through the wire electrode 1 from the upperelectrical feed member 3 that is detected by the upper current detector8; signal I_(DW) of row (e) illustrates the lower detected currentsignal; current difference signal I_(UP) -I_(DW), row (f), illustratesthe difference between said upper and lower detected signals; and signalA/DCLK, row (g), illustrates the timing pulse generated simultaneouslywith the termination or turning off of the gate signal G2. The pulse A/DCLK enables the digital data arithmetic unit 12 to perform thearithmetic operation of converting the incoming signal VD into digitaldata.

Columns A, C and D illustrate the case where machining is carried outunder conditions where the same value is set for the duration of themachining voltage pulse or, in other words, the width of the sparkpulse. The spark of column A is taken to be a spark discharge fired atthe workpieces from a location on the wire electrode midway between theabove-noted feed members 3 and 4 by the absence of any detection signal,i.e., row (f) (I_(UP) -I_(DW)). In reality, the possibility that thelocation is slightly above or below the midpoint, as against right atthe midpoint, is strong as the signal I_(UP) -I_(DW) has been regulatedby the variable resistance VR of the difference amplifier 27. The sparksrepresented by columns C and D occur, respectively, above and below themidpoint. The spark positions can be checked by monitoring the digitaldata PD, as converted and computed by the arithmetic unit 12.

FIGS. 7 and 8 are charts which illustrate characteristic digital data PDfor spark discharge positions output every time a spark discharge pulseis created as described above, or based on the spark discharge pulsessampled as necessary, for example, one pulse out of every three pulses.In either case, in FIG. 7 it is assumed that a 40 mm thick workpiece 2is being machined. In the case of FIG. 7, the data can be classified asindicating steady-state machining. In FIG. 8, the data can be classifiedas machining under non-steady-state conditions, such as shortcircuiting, concentrated spark positions and wire breaking. The graphcharts graduated digital data values (A/D converted spark dischargeposition signals PD) from the arithmetic unit 12 vertically against anumber of sparks (=time) horizontally. As will be clear from the chart,during steady machining (FIG. 7), there is almost no concentration ofsparks, and over the passage of machining time, the plotted points ofspark positions are random and almost evenly scattered and distributedover the whole thickness of the workpiece 2 in the axial direction ofthe wire electrode 1. In the case of a short circuit, or just beforebreaking of the wire electrode 1, or where the sparks tend toconcentrate at a location (FIG. 8), the plotted points of sparkpositions appear to be concentrated at a location of a short circuitpoint along the plate thickness (40 mm) of the workpiece 2. Moreover,despite the existence of a partial scattering to the perimeter, theplotted points are linearly intermittent along the time base. If thisspark distribution continues, a point V where the wire electrode 1breaks, will result. In advance of that breakage, a concentratedlinearization of plotted points as seen in part IV of FIG. 8, mayappear. When the wire electrode 1 has broken, its free ends will come incontact with the workpiece 2 and other electrically conductivestructures and will strike sparks. Thus, as illustrated in FIG. 8, onwire breakage the position of spark points may extend in the axialdirection of the wire electrode 1 beyond the plate thickness of theworkpiece 2. According to the invention, by providing an arrangementwhereby the spark discharge positions can be detected and distinguishedwith high accuracy, the ability to detect and anticipate a break of thewire electrode 1 can also be realized.

FIG. 6 is a waveform chart illustrating the case where the peak holdingcircuit (FIG. 3) is used as the holding circuit 10 for detecting currentsignals between the current detectors 8 and 9 and the spark dischargedetecting means 11, as described above. In FIG. 6, HI_(UP), row (d'), isa peak holding waveshape of the upper detected currents. Similarly,HI_(DW), row (e'), is a lower peak holding waveshape. A signalequivalent to the peak value of detected current signals is held untilthe RESET signal 1, row (h), is delivered. The difference signal HI_(UP)-HI_(DW) (f') is differentially amplified, and a time setting pulse T,row (g'), delayed with respect to the time of gate G2, as shown in FIG.5, is delivered at the time the peak holding difference signal isamplified to allow the digital data arithmetic unit 12 to carry out isconversion operation to output the digital data PD. As described above,when, under predetermined machining conditions, the voltage or sparkpulse width is set relatively wide (in duration) with respect to theamplitude of the spark current, and the amplitude of spark currents IG,row (c), of FIG. 5, increases to its predetermined maximum value by theend of the pulse width, there is of course no need to delay the set timeof the pulse T as described above.

Further, with regard to obtaining the signal VD from the spark positionsignal VC, the signal VC in FIG. 4 is regulated by a variable gaininversion amplifier, consisting of the operational amplifier 31 andother components. For example, comparing the spark resulting from ashort gate signal (e.g., column B of FIG. 5) and the spark resultingfrom a longer gate signal (e.g., column D of FIG. 5), the difference ofdetected current signals are nearly equal: column B(I_(UP)-I_(DW))=column D(I_(IP) -I_(DW)). The signal VC is the signal amplifiedby the difference amplifier 27 of the spark position detecting means.When that signal is converted into the digital data PD by the arithmeticunit 12, the possibility of detecting sparks occurring at the sameposition is high. However, in response to the upper detected currentsignal, row (d), and lower detected current signal, row (e), ofindividual sparks, the ratio of upper (I_(UP)) to lower (I_(DW)) isabout 1:2 for the spark in column B and about 2:3 for the spark incolumn D. In accordance with this data, these would be recognized assparks in different positions; at approximately 67 percent and 60percent, respectively, of the thickness of the workpiece 2, as measuredfrom the workpiece bottom. Thus, there is provided an arrangement asdescribed above, wherein the internal resistance of the transistor TR3is controlled to obtain the amplified and regulated signal VD which isthen converted into digital data indicative of spark positions.

Next, as depicted in FIG. 9, we will describe the inside/outsideworkpiece thickness detector 15, which detects the spark position ofeach spark pulse or sampled spark pulse, and determines whether or notthe spark discharge point is within the thickness of the workpiece 2. Inother words, the detector 15 determines whether or not a spark dischargehas occurred between the electrode and an object other than theworkpiece 2. We will also describe the wire breakage discriminationdevice 16 which generates a wire electrode break signal (DANSEN) to stopthe operation of the wire cut discharge machining device or at least themachining operation. The wire electrode break signal (DANSEN) is outputwhen a determination is made based on the signals from the sparkdischarge position detection device 15 that the wire electrode hasbroken. Specifically, when the detection signals are biased beyond acertain amount towards outside-the-workpiece thickness signals, or whenthe frequency of the outside-the-workpiece discharge position signalsexceed that of the inside-the-workpiece discharge position signals by apredetermined value, or when an abnormal number of detection signalsoccurs, a wire electrode break signal will be output.

The inside/outside workpiece thickness detecting device 15 consists ofthe following: a latching circuit 33 which receives digital datacorresponding to the spark discharge position output by said arithmeticunit 12; adjusters 34 and 35 which preset the digital data value andupper and lower limit value for the spark discharge position,respectively, of the upper half thickness +1/2T and lower halfthickness-1/2T, measured from the center, e.g., usually one-half of thethickness T of workpiece 2; spark position digital data (A) of the mostrecent spark pulse which is output from latching circuit 33; first andsecond upper/lower limit comparators 36 and 37 which compare data (A)with the upper/lower limit thickness digital data values (B) and (B')which respectively correspond to the upper and lower positions from thenormal center position of workpiece 2 as preset by adjusters 34 and 35,respectively; a delay timing pulse generation device 38 which outputs alatching signal, in the form of timing signal CP1, to the latchingcircuit 33 simultaneously with the closing of the gate signal G2 or thestart of the timing pulse T, and which outputs a delay timing pulse CP2,slightly after the timing pulse CP1 is output, in order to generate anoutside/inside workpiece thickness detection signal indicative ofwhether the spark position of the most recently generated spark pulse isinside or outside the thickness of the workpiece A logic circuitreceives signals indicative of comparison results by comparators 36 and37, makes a logical determination upon the generation of delay timingpulse CP2, and outputs an inside workpiece thickness detection signal INif the spark discharge position is inside the upper/lower limit of thethickness of workpiece 2 being machined, and a thickness outsidedetection signal OUT if the spark discharge position is outside of thethickness of the workpiece 2. The detector 15 includes a logic circuitincorporating three AND gates 39, 40 and 41, and one invertor circuit12.

A broken wire electrode discrimination device 16 including a counter 43is arranged so that if the detection signal OUT is generated, it will beinput to count terminal CLK to be integrally counted, whereas if thedetection signal IN is generated, it will be input to reset terminal CLRto reset the integrally counted number. The discrimination deviceutilizes four output terminals Q (A-D), and in this example, outputterminal QA outputs a wear check starting signal to electrical feedmember wear detection device 70 upon input of one thickness outsidedetection signal OUT, and output terminal QB outputs a signal when thecount of thickness outside detection signal OUT reaches a set number(which number is equal to or greater than two), for example a count often. If a signal is output from terminal QB, flipflop 44 is set, wireelectrode breakage signal DANSEN is output to wire spark machiningcontrol device 7, and wire spark machining operation is stopped. Sincethere was a breakage, or at least a determination of a breakage, thebreakage signal DANSEN is also output to feed member wear detectiondevice 80 as a signal to halt or terminate wear detection. Also, outputsignals which are the result of comparisons made by comparators 36 and37, and the signal from output terminal QA of counter 43 are input tothe electrical feed member wear detection device 70 (described below) inorder to determine which of the feed member(s) experienced the wear.After the output of wire electrode breakage signal DANSEN, the wireelectrode is restored by joining or replacing the broken wire electrode,automatically or manually, and other necessary changes and adjustmentsare made to the machining conditions. Thereafter, machining can beresumed with the input of machining resume signal MACHSTART to resetterminal RES of flipflop 44.

The numerical values, such as the above described digital data value Acorresponding to the spark discharge position, which are output byarithmetic unit 12 and latched by the latching circuit, and the digitaldata values B and B' which are preset in accordance to the upper halfthickness +1/2T and lower half thickness -1/2T of the workpiece 2, aresettings where machining conditions are not particularly abnormal,excluding situations where for example, the power transfer from theupper and lower feed members 3 and 4 to the wire electrode ismalfunctioning. In the case where the apportioned ratio of the flow ofspark feed current from the upper and lower feed members to the wireelectrode 1 is 1:0, or even in the case where an uneven feed ratio of5:1 is detected, this information is converted to spark position digitaldata A indicative of an uneven electric supply, concentrated at eitherthe upper or lower feed members and beyond the thickness of theworkpiece. Data A is compared to preset data B and B' and an outsidethickness detection signal OUT is output. Incidentally, even under suchconditions, machining may proceed normally, assuming normal machiningconditions have been appropriately selected, set and adjusted. The valueof digital data PD of the spark discharge position, output by digitaldata arithmetic unit 12 per every spark pulse, is input to terminal A ofcomparators 36 and 37 respectively, and is compared to the data valuesfor the upper and lower limit values, B and B', respectively, whichrepresent the upper and lower thickness from the center of the thicknessof workpiece 2. As a matter of course, the above value input to terminalA will be less than the upper limit value (A<B) and higher than thelower limit value (A>B'). Since each of the comparators 36 and 37 willrespectively operate according to its set conditions, each will output asignal to AND gate 39, which will in turn output a signal to terminalCLR of counter 43 from AND gate 40 when the delay timing pulse CP2 isoutput from pulse generation circuit 38 after a slight delay from thelatching of above compared data A. If an integrated (non-zero) count ofthe signals input to terminal CLK is stored in the counter 43, thatcount will be reset to zero. If no integrated count is stored in thecounter 43, the count will be held at zero and no signals will be outputby the counter 43. The above described detection and determination isrepeated in succession for essentially all of the spark pulses or for apreset sampling of spark pulses which occur during normal wire cutelectrical discharge machining. When the outside/inside workpiecethickness detection device 15 for the spark discharge position outputsonly inside thickness detection signal IN, machining will proceed underconditions illustrated by FIG. 7 with no output from the wire electrodebreakage detection device 16.

Even when machining under normal settings as described above, variousmachining phenomenon may occur in situations where the machining isextended for a long time, where problems or changes in conditions arisein some control system, or where inappropriate conditions exist withinthe preset machining conditions or its balance and the like. FIG. 8shows one example of time (abscissa) distributed condition of digitaldata PD representing spark positions under unusual conditions whereconcentrated sparks or a short circuit condition is occurring randomlyand frequently in the machining gap. The length of the horizontal axisis within a time frame of about 5 ms. FIG. 8 illustrates a seriouscondition where the digital data PD and its plotted points for eachspark pulse suggest that spark concentration and short circuit have beenfrequently occurring in the machining gap from the beginning and animmediate breakage of the wire electrode would not be surprising. Wirecut electrical discharge machines equipped with the present inventionwill strive to achieve and maintain highly efficient machiningperformance by preventing wire electrode breakage from occurring throughthe correction of spark concentrations and short circuits. Adjustmentswill be made to machining conditions, for example, by varying control ofthe mean working current to half or less (including a temporary zerosetting) by controlling the OFF time between voltage pulses, controllingthe machining feed e.g. reducing and stopping the feed rate or backwardfeeding, or varying control of the working fluid supply, particularlythe flushing conditions and flushing situation. One or more of thesecontrols will be used immediately upon a detection of random,intermittent and frequent spark concentration or short circuit, asillustrated in FIG. 8, or upon a detection, by various detection meansand circuits, of precursors to unsteady state machining conditions.

While a wire electrode break accident seldom occurs, when it doeshappen, it is necessary to have prompt detection to activate variouscorrective devices or safeguards, such as those used to stop themachine. According to the present invention, the spark dischargeposition of a sparking pulse, generated by the impression of a machiningpulse between the wire electrode 1 and the workpiece 2 and in between apair of feed members 3 and 4 is accurately detected, converted intodigital data PD=A with the arithmetic unit 12, and output to theinside/outside workpiece thickness detection device 15. This data isthen compared with the digital data B, which is the upper limiting valuecorresponding to the upper side of the thickness of workpiece set in thecomparator 36 of detector 15, and the digital data B', which is thelower limiting value corresponding to the lower side of the thickness ofworkpiece set in comparator 37. A signal is output after logicallydiscriminating whether the spark discharge position of the spark pulseis within the thickness (IN) of the workpiece by satisfying thecondition B>A>B' or whether it is outside the thickness (OUT) and bysatisfying the condition A>B and/or A<B'. If the detector 15 outputs anoutside thickness signal OUT, that OUT signal will be counted by counter43 of the wire electrode breakage discrimination device 16. The count ofthickness outside signal, OUT, by the counter 43 will continue until aninside thickness signal IN is output by detector 15,

A signal is output by the counter 43 when a set count number, forexample, a ten count, is integrally counted. Further, flipflop 44 is setand wire electrode break signal DANSEN is delivered to the machiningcontrol device 7 to enable a control operation such as clamping or atleast a temporary stopping of the travel and movement of the wireelectrode, using, for example, the machining feed drive circuit 7B, orstopping, at least temporarily, the delivery of machining pulses fromthe machining power supply 74 controlling the working fluid supplyingcircuit 7C and the like.

With this embodiment, the time from when the first OUT signal isgenerated to the time when the wire electrode break signal DANSEN isgenerated and a control operation commenced, will be considerablyshorter than about 100 μs. For example, wire break detection, controland the like based on spark position detection, takes place after pausetime control between voltage pulses (where machining pulse OFF time isincreased by a factor of 10), can be performed in a short time e.g. atthe longest about 1 ms or thereabouts. Therefore, by monitoring theposition detection data occurring at or about the time of wire electrodebreakage a break signal (DANSEN) can be generated very soon after thedigital data (PD) representative of spark discharge positions, displaysa characteristic such as area V of FIG. 8, which represents a verticaland random scattering of discharges outside of the workpiece thickness,indicative of the wire electrode breakage.

It is known that at the time of wire electrode breakage, the broken endsegments may move randomly about and come into contact with workpiece 2,adjacent machine areas and other areas--such as the liquid supply guideblock, in which the wire electrode is inserted, preferably in a coaxialmanner with respect to a nozzle for jetting the working fluid. The guideblock incorporates guide members to position the wire electrode, as wellas other parts inside the liquid supply and the guide block structurewhich are combined and integrated into the above guide blocks portion.It is a result of intermittent contact with portions of this structurewhich may result in short circuiting, spark discharges and the like.This is because on wire breakage, the portion of the wire electrodewhich is used for machining, i.e., the portion of between the upper andlower liquid supply and guide blocks, is slack, and the space betweenthe wire electrode and the feed members is such that random contact andbroken contact, in comparison to the steady sliding contact conditionprior to the breakage, may occur. Under such conditions spark positions,determined to be outside the thickness of the workpiece, may result.Moreover, spark position data may be generated which is actuallyindicative of opposing ends of the broken wire electrode contacting eachother or the wire electrode contacting an associated portion of theelectrical feed member.

Over time, even normal machining may cause a wear deformity to the upperand/or lower feed members 3 and 4. In such a case, a large portion ofthe discharge current from a spark pulse generated in the machining areamay flow from the side where the feed member has not deformed from wear,and the digital data representative of such a spark position may bedetected and erroneously determined to be outside the workpiecethickness. Recognition of this condition has led to the next aspect ofthe present invention described below, which provides a wear detectionsystem for diagnosing electrical feed member wear.

FIG. 10 is a longitudinal section diagram illustrating an example of anupper and lower liquid supply and guide block which are respectivelyattached to the upper and lower arms of the machine or to a machinehead. Reference numbers 45 and 45' denote upper and lower guide blockswhich include an insertion hole along the axis of travel of themachining portion of the wire electrode 1. Reference numerals 46 and 46,denote upper and lower liquid supply blocks which, together with 45 and45', form, as a single unit, the passageway for the working fluid.Reference numerals 47 and 47' denote upper and lower nozzles, andreference numerals 48 and 48', respectively, denote upper and lowernozzle holders. Reference numerals 49 and 49' denote lead-in andlead-out members, respectively, each including a draining guide intowhich the wire electrode is inserted, and which form inlet and outletpassages, respectively, for the wire electrode. Reference numerals 50and 50' denote die-shaped upper and lower positioning guide members,respectively, arranged at the end parts of the insertion holes in theupper and lower guide blocks 45 and 45'. Reference numerals 3 and 4denote upper and lower feed members, respectively, arranged to face saidinsertion holes. Reference numerals 51 and 51', respectively, denoteupper and lower current carrying members which adjust the degree ofprojection of the feed members 3 and 4, into the insertion holes, andwhich can also be jointly used as a brush to connect the feed members 3and 4 to one terminal of the machining power supply 7A, usually, theminus terminal. Reference numerals 52 and 52' respectively denoteworking fluid inlet ports to the upper and lower liquid supply and guideblocks. In addition, the feed members 3 and 4 are formed from very hardand wear resistant materials having good electrical conductivitycharacteristics, usually formed by pressuring, compacting and sinteringpowder particles of tungsten carbide using cobalt or the like as abinder.

Geometrically, the electrical feed members 3 and 4 may be pin shapedmembers that are cylindrical or the like, or plate or block-shapedmembers that may have triangular or square cross sections or the like.However, in the illustrated embodiment, the feed members 3 and 4 have agenerally semi-cylindrical shape to allow their top face side (i.e., theside opposite the members 51 and 52') to be oriented towards theinsertion hole so that the face of the feed members intersect the spaceat a right angle, as illustrated in FIG. 10, so that the contact lengthbetween wire electrode 1 and feed members 3 and 4 can be increased, andcontact portion with wire electrode 1 can be changed by varying andadjusting the position of the feed member 3 and 4 to adjust forappropriate friction or the like.

FIG. 11 is a cross sectional diagram of a section perpendicular to theaxis of wire electrode 1, which illustrates one example of a structurefor positioning the upper and lower feed members 3 and 4 in the guideblock sections 45 and 45'. In this figure, reference numerals 53 and 53'denote pressure members for positioning and receiving the feed members 3and 4. In the illustrated embodiment, the upper member 53 has a cylinder54 driving system, and the lower pressure member 53' has a screw andspring driving mechanism 54' to set the receiving positions for the feedmembers 3 and 4. During an automatic insertion or joining operation ofwire electrode 1, the pressure member 53 is automatically controlled bythe cylinder 54, which is moved backward. Reference numerals 55 and 55'are upper and lower energy on-off cylinders which maintain a set slidingcontact condition with wire electrode 1, using energy members 51 and 51'as pistons to drive and adjust the feed members 3 and 4. Referencenumerals 56 and 56', respectively, denote feed actuating bars which areoriented in an opposing contact relationship with the feed members 3 and4 so that each upper and lower feed members 3 and 4 can be driven in theaxial direction and transverse directions with respect to the wireelectrode 1. On the surface of the activating bars, ratchet gear teethare formed at a predetermined pitch, for step feeding in the axialdirection. Feeding pawls 58 and 58' are actuated by feeding cylinders 57and 57' and backward stopper pawls 59 and 59' are engaged. In FIG. 11,reference numerals 68/68' and 60/60' denote swinging fulcrums for thefeed cylinders 57 and 57' and backward stopper pawls 59 and 59'respectively, and reference numerals 61/61' and 62/62' denotecompression springs for each. In addition, reference numerals 63, 64,65, 66 and 67 denote directional control valves for the cylinders 57,54, 55, 55' and 57', respectively.

In the illustrated situation, for example, when it becomes necessary tomove or change the portion of a feed member which contacts the wireelectrode 1 to a new surface area because wear is detected in either ofthe upper or lower feed members 3 and 4, activating signals are input tovalves 63 and 67 via a feed member driving device 7D from wire sparkdischarge control device 7. Valves 63 and 67 are switched or shuttled toa reverse situation from the position illustrated in FIG. 11, andcylinders 57 and 57' are activated. In the illustration, if the pawls 58and 58' are extended, the backward stopper pawls 59 and 59' rotate aboutfulcrums 60 and 60', and after climbing over the crest of the ratchet,drops to the bottom of the next gear tooth and is held there by springs62 and 62'. Further, with the switching of valves 63 and 67, feed pawls58 and 58' allow cylinders 57 and 57' to rotate about the fulcrums 68and 68', respectively, and after climbing over the crest of the ratchet,drops down to the bottom of the next gear tooth. The system is thenprepared for the next feed drive of the actuating bars 56 and 56'. Thediameter of a typical wire electrode 1 used for wire discharge machiningis about 0.2 mmφ or thereabouts, and scratches and the like on the feedmembers 3 and 4 will result from damage and wear caused by the slidingcontact between the wire electrode and feed member. This can becompensated for by moving the feed members 3 and 4, using the feed driveof the actuating bars 56 and 56', about 1 mm at a time or thereabouts torenew the sliding contract surface portion. As provided by the deviceillustrated in FIG. 11, shifting of the feed member can be performedabout 10 times or thereabouts to compensate for feed member wear.

FIG. 12 is a block diagram of the general arrangement of an illustrativeembodiment of a second aspect of this invention which incorporates anarrangement for discriminating when a wire electrode breakdetermination, made on the basis of spark position detection wherein thespark position is determined to be outside the thickness of theworkpiece by the inside/outside detection device 15, is in fact causedby some other reason, e.g., wear of a feed member. Also incorporated inthe embodiment of FIG. 12 is an arrangement for determining anddiscriminating among other causes for spark position data to be outsidethe thickness of the workpiece, specifically whether the cause is feedmember wear.

In explaining the embodiment of this second inventive aspect, as opposedto the first inventive aspect and its illustrations in FIG. 1, when thefirst outside thickness detection signal OUT is output, after an insidethickness detection signal IN is output by the detection device 15, theoutput from terminal QA of counter 43 of the wire electrode breakagedetection device 16 (FIG. 9), and the output signals indicative of thecomparison results from upper and lower limit comparators 36 and 37 ofthe inside/outside thickness detection device 15, are input to the feedmember wear detection device 70. The feed member wear detection device70 generates a wear detection signal MA and an upper and lower weardiscrimination signal U/D MAMO. If after outside thickness detectionsignal OUT is generated, the wire electrode breakage detection device 16does not output wire electrode breakage detection signal DANSEN within aset amount of time, a feed member wear detection signal MAMO and theupper and lower wear detection signal U/D MAMO is generated. Thesedetection signals are input to the machining control device 7 todisplay, notify, and if necessary, output signals to the feed memberdrive device 7D. As explained above with regard to FIG. 11, the drivedevice 7D is operable to more feed members 3 and 4 a very small distanceso that the surface portion of said feed members 3 and 4, which come insliding contact with wire electrode 1, are renewed.

FIG. 13 is a block diagram which illustrates the details of the feedmember wear detection device 70 and wear discrimination device 80. Thewear detection device 70 consists of two RS flipflop circuits 71 and 72,a one shot pulse circuit 73, two invertor circuit 74 and 75, and fourAND gate 76, 77, 78 and 79. Wear discriminating device 80 consists of asignal conditioning device 81 relating to feed member weardiscrimination, and an upper and lower wear discrimination device 82.

The detection of wear of feed members 3 and 4 is done after theinside/outside thickness detection device 15 (FIG. 9) delivers a digitaldata value A relating to a detected spark position wherein either thecomparator 36 determines that the data value A is equal to or greaterthan the upper limiting data B of the preset thickness, or comparator 37determines that the data A value is equal to or less than the lowerlimiting data B' of the preset thickness. As opposed to a situationwhere a signal is output by one of the comparators 36 and 37, if nosignal is output from either, the AND gate 39 does not output a signal.Then, when the delay timing pulse CP2 is output at its proper time fromthe pulse circuit 38, the AND gate 41 outputs an outside thicknessdetection signal OUT. The signal OUT is counted by counter 43 ofdiscriminator 16. Counter 43 outputs a signal from terminal QA toflipflop 71 of the wear detection device 70. In turn, a wear detectionsignal MA is output from terminal Q of the RS flipflop 71 to the signalconditioning device 81 of the wear discrimination device 80 and to theone shot pulse generating device 73. Thus, the one shot pulse generatingdevice 73 outputs a single pulse signal CP3 to AND gates 78 and 79 inresponse to the initial input of a wear detection signal MA. If thedigital data corresponding to the detected position of the sparkdischarge pulse is equal to or greater than the upper thickness limitdata B preset in comparator 36, and at the same time equal to or greaterthan the lower limit data B' of the comparator 37, then a signal equalto or greater than the upper limit is output from AND gate 76. Flipflop72 is set when AND gate 78 outputs an upper and lower wear (or lowerside wear) signal along with the pulse signal CP3. In contrast to theabove, when the digital data A is equal to or less than the lowerthickness limit data B' which is preset in comparator 37, and the data Ais equal to or less than upper limit data B of comparator 36, then theAND gate 77 outputs a signal equal to or lower than the lower limit, andalong with pulse signal CP3, upper and lower wear (of upper side) signalis output from AND gate 79, and the flip flop 72 is reset.

Further, after the wear detection signal MA is output by the flipflop 71of the wear detection device 70 to the wear discriminator 80, the wirebreakage discrimination signal DANSEN from wire electrode breakagediscrimination device 16 is also input to the wear discriminator 80.Since the signal DANSEN indicates a wire electrode breakage, nodiscrimination signal relating to wear of feed members is produced, anda reset signal is output to RS flipflop 71 of the wear detection device70, at which time wear detection operation is immediately terminated andreset. However, if, after the output of wear detection signal MA fromflipflop 71, no wire breakage discrimination signal DANSEN is generatedfor a preset time period (preferably a few ms or thereabouts), thesignal conditioning device 81 will discriminate that the earlier inputwear detection signal MA was based on wear detection of both or at leastone of the feed members 3 and 4. Then, the device 81 will generate awear discrimination signal MAMO, and will output the signal MAMO to themachining control device 7 and also to an upper and lower weardiscrimination device 82. When the wear detection signal MA is generatedas a result of the detection of upper and/or lower wear by weardetection device 70, it is also input to the R or S terminals of RSflipflop 72 via the AND gates 78 or 79 following if the conditions aremet: A>B' but not A<B or A<B but not A>B, respectively. Thereon, an U/DMAMO signal is output to the upper and lower wear discrimination device82. In accordance with the wear discrimination signal MAMO, the upperand lower wear detection device 82 will output the upper and lower weardiscriminating signal, U/D MAMO, indicative of wear on an upper or alower feed member, to the control device 7 in the same manner as above.Further, the machining control device 7 may provide a warning and/ordisplay of feed member wear, in accordance with at least one of the twosignals output from feed member wear discrimination 80. Under certainsettings, the feed member driving device 7D is operated, using eitherautomatic control or manual control by operator or the like subsequentto confirming the display, and the sliding contact feed portion of thefeed member relating to the wire electrode is changed or renewed. Withrespect to the renewed operation of the feed member, programs and thelike with preset machining power supply and machining feed control maybe used to carry out the appropriate control, for example, terminatingmachining operations at once and thereafter gradually resuming normalmachining.

It is noted that detection of wear of feed members 3 and 4 and the upperand lower wear discrimination was performed under conditions where theworkpiece thickness outside detection signal OUT, which caused theoutput from counter terminal QA, was not from contact energy or nearbyspark discharges between the broken wire electrode ends and the like orbetween portions of wire electrode 1 which are outside the machiningzone which may occur on the complete breakage of the wire electrode andsubsequent contact with parts other than the workpiece 2. In this case,the signal OUT resulted from a defective sliding contact between upperfeed member 3 and wire electrode 1, caused, for example, by an excessivewear deformity on the upper feed member 3. The current for the sparkpulse was drawn solely for the lower feed member 4, and this causeddetection and discrimination that the digital data corresponding to thespark position was deemed to be outside the workpiece thickness lowerlimit value. In this situation, it may be determined that a wear existsin a feed member, with such wear determined to have occurred in theupper feed member 3.

As illustrated above, according to the wire cut electrical dischargemachining apparatus of the present invention, the position of the sparkin the axial direction of the wire electrode resulting from a sparkgenerating pulse can be detected as to whether it is inside or outsidethe thickness of the workpiece. When the number or the frequency of thesparks occurring outside the thickness reaches or exceeds a presetlevel, wire electrode breakage discrimination is performed. Detectionand discrimination of wire electrode breakage can be accomplishedquickly and accurately, thus avoiding the occurrence of malfunction ordamage to the workpiece, working fluid nozzle, feeder and wire transportsystems.

In addition, according to the wire cut electrical discharge machiningapparatus of the present invention, when a spark pulse is generatedoutside the thickness of the workpiece, such pulse is detected and alogical discrimination is performed. Where it is not a wire electrodebreakage accident, detection and discrimination for feeder member wearis performed, thereby allowing reliable detection of wear. Further, whenwear discrimination of the wear is performed to discriminate betweenupper and lower feed member wear, it is now possible to provide forfaster response and treatment, and thereby increase machiningperformance.

The foregoing description of a preferred embodiment of the variousaspects of the invention has been presented for purposes of illustrationand description. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiment was chosen and described in order to bestexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

We claim:
 1. A wire break detection system for a wire cutelectrical-discharge machining (WC-EDM) apparatus, said apparatusincluding a renewable wire electrode moveable in its axial directionthrough a working zone of said apparatus wherein a workpiece to bemachined may be positioned, said wire electrode being held under tensionand in contact with upper and lower electrical feed members respectivelydisposed on opposite sides of said working zone, and wherein, duringmachining, said workpiece is spaced from the wire electrode in saidworking zone by a working gap across which machining pulses areimpressed in order to produce spark discharges between said wireelectrode and said workpiece, said detection system comprising:at leastone current detector for detecting the current flowing through at leastone of said electrical feed members; a spark position detecting meansfor amplifying the detected current value from said at least one currentdetector and generating a signal corresponding to the spark dischargeposition along the axial direction of the wire electrode; a digital dataarithmetic unit for converting the spark discharge position signals fromsaid spark position detecting means into digital data; means for settingupper and lower limit values of digital data corresponding to thelocations of the upper and lower surfaces of a workpiece to be machined;means for comparing the digital data representative of the sparkposition with said upper and lower limit values; a spark positiongenerating means for generating a signal indicative of a spark position,said spark position generating means being responsive to a sparkposition digital data value greater than said upper limit value or lessthan lower limit value, to generate a signal indicative of a sparkdischarge outside of said workpiece thickness; and a broken wirediscrimination means, responsive to said signals indicative of the sparkposition being outside the thickness of the workpiece for generating awire break signal.
 2. The wire break detection system according to claim1, wherein said WC-EDM further includes upper and lower wire guidemembers disposed respectively on opposite sides of said working zone,said wire guides being operable to define therebetween the axialposition of said wire electrode as it passes through said working zone.3. The wire break detection system according to claim 1, wherein saidspark position detecting means is operable to generate a position signalfor each spark discharge generated.
 4. The wire break detection systemaccording to claim 1, wherein said spark position detecting means isoperable to generate a position signal for a predetermined sampling ofspark discharges.
 5. The wire break detection system according to claim1, wherein said upper and lower limit values of digital data are setusing the center of said workpiece as a reference point, the center ofsaid workpiece being located essentially midway between said upper andlower limit values.
 6. The wire break detection system according toclaim 1, wherein said machining pulses are generated using a gate pulsesignal, further comprising means for generating a timing pulse signalslightly delayed with respect to said gate pulse signal, and whereinsaid means for generating the signal indicative of a spark positionoutside of the workpiece is further responsive to said timing pulsesignal to enable said position signal to be generated.
 7. The wire breakdetection system according to claim 1, wherein said spark positiondetection means includes means for amplifying the detected current fromsaid at least one current detector.
 8. The wire break detection systemaccording to claim 1, wherein said machining pulses are generated bymeans of a gate pulse signal, further comprising means for generating atiming signal, slightly delayed in time from the termination of saidgate pulse signal, said timing signal being input to said digital dataarithmetic unit for timing the conversion of said spark dischargeposition signal into said digital data.
 9. The wire break detectionsystem according to claim 8, wherein said spark position detecting meansincludes a means for amplifying the detected current from said at leastone current detector, said amplifying means including means for varyingthe amplification factor in accordance with predetermined machiningconditions.
 10. The wire break detection system according to claim 1,wherein said at least one current detector comprises a pair of currentdetectors, one associated with each of said upper and lower electricalfeed members.
 11. The wire break detection system according to claim 10,wherein said machining pulses are generated by means of a gate pulsesignal, further comprising a sampling and holding means for receivingthe current signals from said current detectors and latching saidcurrent signals synchronously with said gate pulse signals.
 12. The wirebreak detection system according to claim 11, wherein said sparkposition detection means includes means for amplifying the currentsignals from said current detectors.
 13. The wire break detection systemaccording to claim 11, wherein said sampling and holding means comprisesa peak current holding circuit means for latching the maximum value ofthe detected current from each of said current detectors and forgenerating an output signal synchronously with the termination of saidgate pulse signal.
 14. The wire break detection system according toclaim 13, wherein said sampling and holding means is operable forlatching the maximum value of the detected current for each sparkdischarge.
 15. The wire break detection system according to claim 13,wherein said sampling and holding means is operable for latching themaximum value of the detected current for a predetermined sampling ofspark discharges.
 16. The wire break detection system according to claim13, wherein said peak holding circuit means comprises first and secondpeak holding circuits responsive to said upper and lower currentdetectors, respectively, and wherein spark position detection meansinclude a difference amplifier operable for extracting a signalindicative of the difference between the peak current values held bypeak holding circuits to thereby generate a spark position signal. 17.The wire break detection system according to claim 16, furthercomprising means for generating a timing signal slightly delayed in timewith respect to the termination of said gate pulse signal, said timingsignal being input to said digital data arithmetic unit for timing theconversion of said difference signal into said digital data.
 18. Thewire break detection system according to claim 17, further comprising anamplifier means for amplifying said difference signal, said amplifiermeans having means for varying the amplification factor thereof, wherebysaid spark position signal is amplified in accordance with predeterminedworking conditions.
 19. The wire break detection system according toclaim 1, wherein said spark position generating means is responsive to aspark position digital data value less than said upper limit value andgreater than said lower limit value to generate a signal indicative of aspark discharge between said workpiece and wire electrode.
 20. The wirebreak detection system according to claim 19, wherein said broken wirediscrimination circuit comprises a counter for counting up the number ofsaid signals indicative of a spark discharge outside the thickness ofthe workpiece for generating said wire break signal when said countreaches a predetermined value.
 21. The wire break detection systemaccording to claim 20, wherein said counter is operable, in response toa signal indicative of a spark discharge between wire electrode andworkpiece, to reset said count to zero.
 22. The wire break detectionsystem according to claim 20, wherein said WC-EDM apparatus furthercomprises a control device for controlling said WC-EDM apparatus, andwherein said wire break signal, upon generation, is output to a controldevice for controlling said WC-EDM apparatus.
 23. The wire breakdetection system according to claim 19, further comprising an electricalfield member wear detecting means, responsive to the generation of saidsignal indicative of a spark discharge outside of the workpiecethickness after the generation of said signal indicative of a sparkdischarge between said wire electrode and said workpiece, to generate asignal indicative of wear of at least one of said upper and lowerelectrical feed members.
 24. The wire break detection system accordingto claim 23, further comprising a wear discrimination means, responsiveto said wear detecting means, to generate a feed member wear signal inresponse to a wear detection signal if no broken wire signal isgenerated by said broken wire discrimination means within apredetermined time after generation of said wear detection signal. 25.The wire break detection system according to claim 24, wherein saidWC-EDM further comprises a display and said wear discrimination means isoperable to drive said display to indicate said feed member wear status.26. The wire break detection system according to claim 24, whereinduring operation of said WC-EDM, contact between said upper and lowerelectrical feed members and said wire electrode is a sliding contact,and further comprising means, responsive to said wear detection signal,for adjusting the position of at least one of said electrical feedmembers relative to said wire electrode whereby a new site of slidingcontact is established between said at least one electrical feed memberand said wire electrode.
 27. The wire break detection system accordingto claim 26, wherein said adjusting means comprises upper and loweractivating members respectively positioned to contact said upper andlower electrical feed members and operable to be driven, in response tosaid wear detection signal, to thereby move its associated electricalfeed member.
 28. The wire break detection system according to claim 24,wherein said means for comparing is operable to generate a first signalif said spark position digital data is less than said lower limit valueand a second signal of said spark position digital data is greater thanupper limit value, and wherein said feed member wear detecting meanscomprises a logic circuit, responsive to said first and second signalsfor generating an up/down signal indicative of whether the wear signalis a result of wear of said upper electrical feed member, said lowerelectrical feed member, or both.
 29. The wire break detection systemaccording to claim 28, wherein said WC-EDM further comprises a displayand said wear discrimination means is operable to drive said display toindicate said feed member wear status and which of said upper and lowerfeed members is worn.
 30. The wire break detection system according toclaim 28, wherein, during operation of said WC-EDM, the contact betweensaid upper and lower electrical feed members and said wire electrode isa sliding contact, and further comprising means responsive to saidup/down signal for selectively adjusting the position of said upperelectrical feed member, said lower electrical feed member, or both,relative to the wire electrode whereby a new site of sliding contact isestablished between said selected upper electrical feed member, lowerelectrical feed member, or both, and said wire electrode.
 31. The wirebreak detection system according to claim 30, wherein said adjustingmeans comprises upper and lower activating members respectivelypositioned to contact said upper and lower feed members and respectivelypositioned to said up and down signals to move its associated electricalfeed member.
 32. A method of detecting electrode wire breakage in awire-cut electroerosion machining apparatus having a renewable wireelectrode moveable in its axial direction through a working zone of saidapparatus wherein a workpiece to be machined may be positioned, saidwire electrode being held under tension and in contact with upper andlower electrical feed members, respectively disposed above and belowsaid working zone, said workpiece being spaced from said wire electrodein said working zone by a working gap across which electrical machiningpulses are impressed, said method comprising the steps of:detectingcurrent flowing through at least one of said electrical feed members asa result of a machining pulse; amplifying the detected current andgenerating a spark position signal based on said amplified detectedcurrent signal; converting said spark discharge position signal intodigital data; comparing said digital data with data representative ofthe upper and lower surfaces of said workpiece; and generating a wirebreak signal when said spark position data falls outside of saidworkpiece thickness.
 33. The method according to claim 32, wherein thestep of detecting further comprises the step of detecting a valveindicative of the peak current detected during a machining pulse. 34.The method according to claim 32, wherein the step of detecting furthercomprises detecting the current flowing through each of said upper andlower feed members and the step of amplifying further comprisesdifferentially amplifying the detected current from said upper and lowerfeed members to thereby generate said spark position signal.
 35. Themethod according to claim 32, wherein the step of generating a brokenwire signal further comprises counting the number of times the sparkposition data indicates a spark discharge position outside of theworkpiece thickness and generating said wire break signal when saidcount reaches a predetermined value.
 36. A method of detectingelectrical feed member wear in a wire-cut electroerosion machiningapparatus having a renewable wire electrode moveable in its axialdirection through a working zone of said apparatus wherein a workpieceto be machined may be positioned, said wire electrode being held undertension and in contact with a portion of an upper and a lower electricalfeed member, respectively disposed above and below said working zone,said workpiece being spaced from said wire electrode in said workingzone by a working gap across which electrical machining pulses areimpressed, said method comprising the steps of:detecting current flowingthrough said electrical feed members as a result of a machining pulse;differentially amplifying the detected current flowing through each ofsaid wire feed members and generating spark position signal data;comparing said spark position signal with data representative of theupper and lower surfaces of said workpiece; and generating an electricalfeed member wear signal when said spark position signal data fallsoutside of said workpiece thickness after generation of spark positionsignal data indicative of a spark discharge positioned within theworkpiece thickness.
 37. The method according to claim 36, wherein thestep of generating an electrical feed member wear signal furthercomprises the steps of counting the number of consecutive times saidspark position signal data indicative of a spark discharge outside ofsaid workpiece thickness is generated and generating a feed member wearsignal if a predetermined count is not reached within a predeterminedtime.
 38. The method according to claim 37, further comprising the stepof adjusting the positioned electrical feed member in response to a feedmember wear signal, whereby a new portion of said electrical feed membercontacts said wire electrode.
 39. The method according to claim 36,further comprising the step of discriminating whether the spark positiondata used to generate feed member wear signal is indicative of wear ofthe upper electrical feed member, lower electrical feed member, or both,and issuing a discrimination signal indicating the worn electrical feedmember or members.
 40. The method according to claim 39, furthercomprising the step of adjusting the position of the discriminatedelectrical feed member, whereby a new portion of said indicatedelectrical feed member contacts said wire electrode.
 41. The methodaccording to claim 39, wherein said step of discriminating furthercomprises generating a first signal if said spark position signal datais greater than a first value indicative of the top surface of saidworkpiece, and generating a second signal if said spark position signaldata is less than a second value indicative of a bottom surface of saidworkpiece, and generating said discrimination based on said first andsecond signals.